The present invention relates to a method of metal layer formation for forming a metal layer on a surface of a porous resin layer and to a metal foil-based layered product which can be obtained by the method. The invention is useful especially for forming an insulating layer or wiring layer for wiring boards for high-frequency use.
With the recent trend toward higher information processing rates and higher communication wave frequencies in information/communication appliances, the wiring boards on which electronic and other parts are to be mounted are also required to have suitability for high frequencies. For example, the insulating layers in such wiring boards are required to have a low permittivity and a low dielectric dissipation factor at high frequencies so as to attain excellent high-frequency transmission characteristics and excellent low-crosstalk characteristics.
The reasons for that are as follows. In the circuit of a wiring board, an energy loss in transmission, which is called a transmission loss, occurs. This transmission loss is proportional to the product of the frequency f of the signal, the xc2xd power of the dielectric constant ∈, and the dielectric dissipation factor tan xcex4 of the material. Because of this, materials to be used in wiring boards for higher frequencies f are especially required to have a low dielectric constant ∈ and a low dielectric dissipation factor tan xcex4. Furthermore, materials having a low dielectric constant ∈ are desired for high-frequency applications also from the standpoint that the rate of signal transmission is inversely proportional to the xc2xd power of dielectric constant ∈.
In the method which has been generally employed for forming such an insulating layer having a low permittivity and a low dielectric dissipation factor, a resin material which itself has a low permittivity is used. Known examples of such low-permittivity resin materials include fluoropolymers such as polytetrafluoroethylene and polyimide resins.
On the other hand, there is a technique for forming an insulating layer having a lower permittivity than the resin material itself constituting the layer. This technique comprises forming the insulating layer so as to have a porous structure. For example, JP-A-62-279936 proposes a process for producing a metal foil-based layered product for high-frequency use which comprises forming a porous precursor layer on a metal foil by a wet film-forming method using a solution containing a poly(amic acid) and then imidizing the polymer to thereby form a porous polyimide layer. This metal foil is processed mainly by wet etching in a later step to form a circuit pattern. Thus, the metal foil is used as a wiring layer. In order for such a metal foil-based layered product to be used as a double-sided wiring board, it has been necessary that a metal layer should be formed, prior to the etching step, on the layered product on the side opposite to the metal foil by laminating another metal foil to that side or forming a metal layer on that side by plating.
However, it was found that when the metal foil-based layered product described above is subjected to electroless plating using a plating solution so as to form an undercoat for a metal layer on the surface of the porous layer, then the plating solution infiltrates into pores of the porous layer. This plating solution infiltration not only makes it difficult to obtain a satisfactory undercoat but also results in difficulties in removing the plating solution which has infiltrated. In addition, there is the possibility of posing problems such as wiring pattern short-circuiting.
JP-A-2000-319442 proposes a process for producing a metal foil-based layered product which comprises forming a porous polyimide layer on a substrate other than metal foils in the same manner as described above and then superposing a metal layer on one or each side of the resultant coated substrate through a heat-resistant adhesive layer. The examples of this heat-resistant adhesive which are specifically disclosed in that reference include films (20 xcexcm thick) formed from thermosetting resins.
Although the method in which a metal layer is superposed through an adhesive layer is effective in avoiding the problems caused by plating solution infiltration, it has a problem that the insulating layer obtained has a permittivity increased by a value corresponding to the thickness of the adhesive layer. Furthermore, there are cases where the laminating crushes pores in the porous layer, resulting in increasing the density of the porous layer and thereby heightens the permittivity of the insulating layer.
One object of the invention is to provide a method of metal layer formation which can satisfactorily eliminate the problems caused by plating solution infiltration and is sufficiently effective in reducing the permittivity of an insulating layer.
Another object of the invention is to provide a metal foil-based layered product which can be obtained by the method.
The present inventors made intensive investigations on methods for forming a metal layer on a surface of a porous resin layer, in order to accomplish those aims. As a result, it has been found that those objects can be accomplished by using a porous resin layer having a dense layer as a surface part thereof and by forming a thin metal film beforehand on the surface of the dense layer by a dry process. The invention has been thus completed.
The invention provides a method of forming a metal layer on a surface of a porous resin layer which comprises: a step in which a porous resin layer having a dense layer as a surface part thereof is used as the porous resin layer and a thin metal film is formed on the surface of the dense layer by a dry process; and a step in which a metal film is formed on the surface of the thin metal film by plating. The term xe2x80x9cdense layerxe2x80x9d herein means a part which can be called a skin and has a lower porosity than the other part (the porosity ratio (porosity of the skin/that of the other part) is 0.25 or lower). Whether or not a porous resin layer has such a dense layer can be judged based on an examination of the surface or a section of the porous resin layer with an electron microscope.
In this method, the porous resin layer to be used is preferably one formed over a metal foil directly or through a resin film layer.
In forming the thin metal film, it is preferred that a thin metal film made of a metal or alloy comprising at least one member selected from the group consisting of chromium, titanium, platinum, palladium, and nickel be first deposited in a thickness of 5 nm or larger before forming the thin metal film by a dry process.
The invention further provides a metal foil-based layered product which comprises a metal foil, a porous resin layer formed over the metal foil directly or through a resin film layer, and a metal layer formed on the surface of the porous resin layer, wherein the porous resin layer has a dense layer as a surface part thereof and has a thin metal film formed on the surface of the dense layer by a dry process.
In this layered product, the thin metal film preferably comprises a layer made of a metal or alloy comprising at least one member selected from the group consisting of chromium, titanium, platinum, palladium, and nickel.
The method of metal layer formation of the invention has the following advantages. Since a porous resin layer having a dense layer as a surface part thereof is used, a thin metal film can be satisfactorily formed on the surface thereof by a dry process. This thin metal film has a barrier effect in plating, whereby the problems caused by plating solution infiltration can be satisfactorily eliminated. Since the dense layer is not nonporous although it is dense, the effect of reducing permittivity is higher as compared with the case where an adhesive or the like is interposed. Furthermore, neither the step of forming a thin metal film nor the step of forming a metal film especially necessitates pressing in the thickness direction. Consequently, this method is advantageous also from the standpoint of attaining a reduced permittivity by preventing compression.
In the case where the porous resin layer is one formed over a metal foil directly or through a resin film layer, a layered product having a metal layer on each side can be produced while satisfactorily preventing plating solution infiltration from occurring on the back side. Furthermore, the insulating layer of this layered product has a sufficiently reduced permittivity.
In the case where a thin metal film made of a metal or alloy comprising at least one member selected from the group of metals shown above is first deposited in a thickness of 5 nm or larger in the thin-metal-film formation described above, higher adhesion between the metal layer and the porous resin layer can be obtained as compared with the case where a thin copper film is directly formed by a dry process. Moreover, since this metal deposition is conducted in a thickness of 5 nm or larger, an undercoat layer suitable for diminishing the surface roughness of the dense layer can be formed.
On the other hand, the metal foil-based layered product of the invention has the following advantages. Since the porous resin layer has a dense layer as a surface part thereof and has a thin metal film formed on the surface of the dense layer by a dry process, the problems caused by plating solution infiltration can be satisfactorily eliminated as described above. In addition, the effect of reducing the permittivity of the insulating layer can be sufficiently obtained.
Furthermore, in the case where the thin metal film comprises a layer made of a metal or alloy comprising at least one member selected from the group of metals shown above, higher adhesion between the metal layer and the porous resin layer can be obtained as compared with the case where a thin copper film is directly formed by a dry process.